1. Field of the Invention
The invention relates to a traction drive having torque-transmitting toroidal discs and power rollers with infinitely variable ratio characteristics.
2. Background Art
In a continuously variable transmission mechanism in a powertrain, the torque ratio can be changed without the usual shift between defined torque ratios as in the case of a conventional geared transmission. One class of continuously variable transmissions comprises traction drives with toroidal friction discs that register with power rollers for transmitting torque in a torque flow path from a driving member to a driven member. As in the case of other classes of infinitely variable transmissions, a toroidal drive of this kind makes it possible to reduce throttle losses and increase engine operating efficiency as the engine operates close to its minimum brake specific fuel consumption point.
Torque transfer by the friction discs and the power rollers occurs through a lubricated contact patch. Shear forces in the lubricating oil at the contact patch establish a driving torque flow between the driving member and the driven member. An example of a toroidal drive of this kind can be seen by referring to U.S. Pat. No. 5,052,236.
In the case of the traction drive disclosed in the '236 patent, the transmission ratio is changed by adjusting the spacing of the rotary axes of the power rollers relative to the rotary axis of the friction discs. This creates a misalignment of the tangential force vectors across the contact patch. Vector force components of the tangential forces create a roller tilting moment. The tilting moment adjusts the angularity of the rollers.
In a traction drive of the kind shown in the '236 patent, a reaction force is created as torque is transmitted through the drive. The reaction force is opposed by hydraulic control pistons, which create forces on the rollers to maintain their position following angular adjustment of the rollers during a ratio change. These reaction forces, which are relatively high, require a high control effort that can exceed 5,000 N per roller. Such designs create a design challenge because of the sensitivity of the relationship between transmission ratio and the hydraulic pressure acting on the hydraulic control pistons.
In a traction drive of the kind disclosed in the '236 patent, the rollers are supported by trunnions on a trunnion yoke. The transmission ratio is changed as the effective torque input radius and the effective torque output radius at the contact patches for the discs are changed due to an adjustment of the roller orientation. This adjustment is achieved by adjusting the yoke to laterally offset the roller axis relative to the disc axis, which creates a moment. This moment causes the rollers to tilt to a new ratio. When that ratio is achieved, the offset is eliminated, and the force component creating the tilting moment returns to zero.
In the case of the transmission disclosed in U.S. Pat. No. 6,575,869, the roller trunnions are offset about a gimbal axis that is parallel to the disc axis. This creates a steering moment that adjusts the inclination of the rollers. The control effort on the control pistons thus is isolated from load-bearing requirements typical of traction controls of the kind disclosed in the '236 patent. Because of the reduced forces, the rollers can readily be packaged in a group of three within each toroidal cavity. The stability of the control system thus is enhanced, and the response of the traction drive to a ratio control effort is improved. The reaction forces are transmitted to a gimbal frame, rather than to the yoke that supports the rollers. The frame can be inclined about a pivot axis that passes through two fixed gimbal supports. The inclination of the frame creates an angular offset for the rollers. This offset generates a moment that produces a change in ratio. The control forces essentially are decoupled from the force required to support the rollers.
The subject matter of U.S. Pat. No. 6,575,869 is incorporated by reference in this application.